The present invention relates generally to the field of frequency selection elements, and more particularly to a thin film resonator and method.
Televisions and radios as well as cellular phones and other wireless devices all transmit and/or receive radio frequency signals. Televisions and radios, for example, receive programming from a number of stations in the form of radio frequency signals that are transmitted by the stations. Cellular phones and other two-way wireless communication devices communicate with a base station by both transmitting and receiving radio frequency signals. The radio frequency signals include voice traffic for a wireless telephone connection or data traffic for a wireless Internet or other network connection.
Televisions, radios, cellular phones and other wireless devices are each assigned to different radio frequencies to allow simultaneous operation of the devices within an area. Television, for example, receives signals within the 55 to 800 megahertz (MHz) range while radio receives signals within the 530 to 1,700 kilohertz (kHz) range for AM and within the 88 to 108 megahertz (MHz) range for FM. Cellular phones, in accordance with U.S. standards, operate in the 900 and 1800 megahertz (MHz) range.
Televisions, radios, cellular phones, and other wireless devices each use radio frequency filters to separate out unwanted radio frequency traffic from a desired signal, or channel. In particular, televisions and radios use a number of filters to form a tuner that allows each of the received stations to be selectively tuned. Cellular phones operate at a preset frequency range and include filters dedicated to that frequency range. In each case, the filters discriminate between signals based on frequency diversity to provide a stable signal for use by the receiving device.
Radio frequency filters based on resonators are constructed from pairs of inductors and capacitors arranged in parallel, from crystal resonators and from thin film resonators. The inductor and capacitor configuration resonates in a broad range and therefore provides low quality signal discrimination. Crystal and thin film resonators, on the other hand, resonate in a narrow range and therefore provide high quality signal discrimination.
Crystal resonators include a crystal positioned between a pair of posts. Although crystal resonators provide high signal discrimination, they are limited to applications below 500 megahertz (MHz) due to crystal thickness limitations. As a result, crystal resonators are not suitable for cellular and other lower ultra high frequency (UHF) applications in the 300 to 3000 megahertz (MHz) range.
Thin film resonators are formed on a substrate that includes an acoustic reflector. The acoustic reflector may be formed by an air-gap or a number of reflecting layers. The thin film resonator includes a piezoelectric layer positioned between two electrodes. The piezoelectric layer may comprise zinc oxide (ZnO). Zinc oxide surface acoustic wave (SAW) devices have been developed as thin films on an insulator. TV-IF filters are produced as zinc oxide on glass.
The piezoelectric layer has a thickness that is equal to half the target wavelength for the resonator in order to provide the proper resonance in the resonator. At 900 megahertz (MHz), a half-wavelength is 3.5 microns. Because the piezoelectric layer can only be formed at a slow rate of about 5 microns per hour due to processing limitations, thin film resonators are time consuming and expensive to fabricate. In addition, the substantial thickness of the piezoelectric layer for lower UHF applications causes internal stress within the resonator which leads to warping, bubbling, and cracking defects.
The present invention provides a thin film resonator and method that substantially reduces or eliminates disadvantages and problems associated with previously developed systems and methods. In particular, the conventional piezoelectric layer is replaced with a sandwich layer of piezoelectric and non-piezoelectric material that can be quickly deposited to provide a low cost and high performance thin film resonator.
In accordance with one embodiment of the present invention, a thin film or other suitable acoustic resonator comprises a first electrode and a second electrode substantially parallel to one another. An intermediate layer is disposed between and coupled to the first and second electrodes. The intermediate layer includes a first piezoelectric layer, a second piezoelectric layer, and a spacer layer disposed between the first piezoelectric layer and the second piezoelectric layer. The spacer layer has an acoustic impedance substantially the same as that of the first and second piezoelectric layers and comprises a disparate material.
More specifically, in accordance with a particular embodiment of the present invention, the spacer layer has a coefficient of thermal expansion substantially the opposite of that of the first and second piezoelectric layers. In addition, the spacer layer may be designed to offset the thermal expansion of acoustic reflectors supporting the resonator.
Technical advantages of the present invention include providing an improved acoustic resonator and improved filters employing the acoustic resonators. In particular, the acoustic resonator includes a sandwich of piezoelectric and non-piezoelectric material between the electrodes. The non-piezoelectric material is substantially uniform in thickness and readily formed during fabrication. As a result, thin film and other suitable acoustic resonators may be produced at low-cost.
Another technical advantage of the present invention includes providing an ultra high frequency (UHF) acoustic resonator. In particular, the non-piezoelectric spacer has a substantially uniform thickness over a wide range. As a result, the thickness of the resonator may be substantially increased to support cellular phone and other UHF applications.
Yet another technical advantage of the present invention includes providing a stable acoustic resonator. In particular, the spacer layer may be formed of a material having an opposite thermal expansion characteristics of the piezoelectric layer and/or the acoustic reflector. As a result, stability of the resonator is increased and the device may be configured to be insensitive to temperature within an operational range.
Still another technical advantage of the present invention includes providing an improved on-chip filter. In particular, a thin film resonator is provided that may be readily fabricated directly onto a substrate. The resonator may be combined with other resonators to form an on-chip filter and on-chip filters combined to form a single-chip transceiver.
Other technical advantages of the present invention will be readily apparent to one skilled in the art from the following figures, description, and claims.